1. Field of the Invention
This invention relates to a lidding for containers.
2. Description of Related Art
As packaging technology has progressed and the desire for security and wholesomeness of packaged products for human intake has increased, improved lidding for containers has been developed, comprising a substrate and sealant which is heat sealed to the container. This seal is generally accomplished by placing the lidding on the container to cover its opening and applying heat and pressure through the lid to soften the sealant sufficiently to form the seal between the lidding and the lip of the container surrounding its opening. Unfortunately, the seal is often so strong that the lidding is difficult to remove, requiring puncturing of the lidding with a sharp instrument to enable at least the central part of the lidding to be torn away, often leaving portions of lidding still adhered to the lip surrounding the container opening.
Ideally, the lidding should provide a seal to the container that is simultaneously strong enough to provide a secure closure to the container yet also be weak enough to be easily removed from the container such that there is no residue left on the container lip.
The difficulty in obtaining this ideal condition arises from the nature of the container sealing process and the response of existing lidding sealants, notably copolymers of ethylene and an unsaturated ester such as vinyl acetate or methyl acrylate, to this process. The seal strength of existing sealants have been either too low with respect to many of the common container materials or too sensitive to the sealing temperature. In the latter case, as seal temperature is increased, the seal strength sharply increases, leading to the loss of lidding peelability. Attempts to decrease seal strength by decreasing sealing temperature invariably jeopardizes the integrity of the seal.
Compounding the problem of seal strength temperature sensitivity is the problem of seal temperature variation inherent in the sealing process. In common sealing types of operations, the interface is heated by exposure of the exterior of the lidding material to a heated bar or platen. The temperature at the interface depends upon the temperature of the heated platen, the thickness of the lidding and its ability to conduct heat and the length of time that the platen contacts the lid. It is economically advantageous to attempt to seal the lid to the container as fast as possible. Thus, it is commonly found that sealing operations use very hot platens and very short contact times. Small changes in contact times or fluctuations in the thickness of the lidding can dramatically affect the temperature of the interface and, thus, the resultant strength of the seal. Changes in the temperature of the platen can also affect the strength of the seal. While technology has been developed to provide accurate control of platen temperature, equipment that is older or has not been adequately maintained may result in substantial variation in platen temperature. While providing the heat to achieve the seal, the platen also applies pressure to the lidding to obtain intimate contact between the sealant and the container.
Another disadvantage of existing sealants is that the peel strength to containers of different materials, e.g., of polyethylene, polypropylene, polyester, polystyrene, varies just because of the varying ability of the sealants to adhere to these different materials. Thus, sealants of different compositions have been required depending on the container material involved.
Thus, it is clearly desirable for a sealant to exist which has greater universality of application, i.e., adheres to a wide variety of materials, which is relatively insensitive insofar as peel strength is concerned to heat seal temperature variations, and which provides seal characteristics whereby the seal has both integrity to protect the container contents and easy peelability.
Numerous polymer-based adhesives are available for bonding two layers of dissimilar materials together such as by extruding molten adhesive into the nip formed by converging films of these layers. The rolls which form this nip force the molten adhesive against both films and thereby bond them together via the adhesive. The heat required for forming this bond is provided by the molten adhesive which is controllable via temperature control of the extruder melting the adhesive and forcing it into the nip between the films. This type of adhesive bonding is called extrusion lamination and the purpose of this bonding is to prevent peelability, i.e., to prevent the separation of one film from the other. This result is achieved, in part, by the high temperature used in extrusion lamination, as established by the molten adhesive, as compared to the lidding sealing process. Generally, the lidding sealing process will be carried out at a platen temperature which is usually at least 50.degree. F. less than the typical extrusion melt temperature for a resin of the same composition.
Research Disclosure 27 770 (May, 1987) discloses various polymers and blends thereof used for extrusion lamination of polypropylene to either aluminum foil or a polyvinylidene chloride film. Blends of acrylate copolymers with terpolymer are disclosed as offering a better combination of simultaneous bondability to aluminum foil and other substrates than do blends with ethylene/acid copolymers. No mention is made in this reference of applying any of these extrusion lamination adhesives to lidding utility or to the special conditions and problems unique to this utility as described above, including the need to achieve a seal between lidding and container at a sealing temperature range substantially less than the extrusion melt temperature of the adhesive, where the seal has both integrity and peelability.
U.S. Pat. No. 4,680,340 discloses an approach to solve the problem of the need for seal integrity and easy peelability by having the sealant consist of a blend polymer such as ionomer or ethylene/vinyl acetate copolymer of melt flow index less than 5 with a polymer such an LDPE, ethylene/vinyl acetate copolymer, and acid-modified ethylene/vinyl acetate copolymers such as BYNEL.RTM. available from Du Pont having a melt flow index greater than 20. This variation in melt flow index of the two polymer components of the blend together with the selection of components for the blend results in low cohesive strength of the sealant. The easy peel characteristics of this sealant occurs by cohesive failure of the sealant. The disadvantage of the cohesive failure approach is that residue can be left on the lip of the opening of the container rather than leaving a smooth, clean appearing container lip surface when the lidding is removed.